A Newt's Fully Illuminated Field In Degrees

In ANSI C by David Mark North

This is the .c source of a single file, single function program to
take terminal input and calculate the Fully Illuminated Field of view
of your newtonian telescope (or the one you're planning). This
measure is normally given in inches (a meaningless dimension), where
this program delivers degrees of field (or fractions thereof, normally).

This is much more useful, especially for the planetary designer
shooting for optimization. It can tell you if the illuminated zone is
large enough to take advantage of your smallest "high power" eyepiece
in a very simple fashion: if you know the real field of the eyepiece,
you can tell how much of that field (portion of the center) will be
"using the whole mirror" by comparing the two fields. If the
telescope's field is larger than the eyepiece's, you are getting full
use of the mirror across the entire field.

The compressed source file (fif.tgz) is here.

Anyone who can download and compile ANSI C should please do so,
on any available platform. Beware the curious demands of
vc++ on windows, which wants me to do strange things with the header
files. On Mac OS9, there's a little change to drop the kill sequence
so Mac folks can use Command-Q or File/Quit as is normal. On OSX, of
course, it just runs like any *nix terminal app.

The header math.h is not used because it sometimes requires a cc
-lm flag (c++ is smarter about this for some reason) and I wanted a
monolithic C version (max compatibility). The arctangent function
isn't really necessary for reasonable accuracy.

Bug reports to me.

To use the program, you must know your primary diameter, focal
ratio, smallest width of your secondary (minor axis, the size usually
given) and your intercept distance (from the center of your tube
and/or secondary mirror to the focus -- about where the field stop of
an eyepiece sits when in focus).

Don't be too worried about being within a hundredth of an inch or
something nuts like that; just get as close as you can. This is not
an exact solution for a number of reasons, not the least of which is
some curiousities about secondary placement and how it intercepts the
light cone.

The program is heavily commented and more or less explains itself.
It is not meant to be a guide to style, or an example of great
work. It is meant to be small, short, and maximally portable to
anyone with a c compiler and a terminal or console.

You should be able to download and build it yourself with no problem.
Just move it to whatever directory you wish, then type
"cc -o fif fif.c" (forget the quotes) and type ./fif if all goes well.

David North

Copyright 1999 David Mark North http://www.timocharis.com

This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2, or (at your option)
any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
Or, as of the time of this writing, consult:

First published 1999